Atomic and molecular beam machines are powerful, widely used devices in the laboratory study of atomic and molecular properties, but they also find practical application in devices such as portable atomic frequency and time standards. In this latter application, they are integral parts of precision navigation systems and frequently are used in highly dynamic or space environments. As the discussion throughout this application applies equally to most atomic and molecular materials from which one might form a beam, the two terms ("atom" and "molecule") will be used interchangeably throughout.
FIGS. 4 and 5 represent well known prior art ovens. FIG. 4 shows the type of stable oven that would be used in a laboratory beam machine which does not need to operate over a long time period. The working material is contained in a heated chamber A and some of the vapors are allowed to escape through a small hole. The expanding cloud of vapor is intercepted by a collimator B which allows atoms with the correct trajectory to pass down the beam line. The total amount of material emitted through the oven hole can be shown to be: EQU Q.sub.o =1/4nvA.sub.s
where n is the number density of atoms or molecules in the oven chamber, v is their mean thermal velocity and A.sub.s is the area of the source hole. If the collimator hole can be characterized by a radius, r, separated from the oven hole by a distance, L, then the material emitted into the beam can be shown to be: ##EQU1## Thus, if L is very much larger than r, the effect of the collimator is substantially to reduce the total amount of material injected into the beam machine without affecting the onaxis beam flux.
The problem with this oven is the excessive amount of material which leaves the oven chamber but does not contribute to the beam. This material must be trapped behind the collimator. It cannot be allowed to find its way into the beam area or to plug the collimator.
The oven shown in FIG. 5 was developed in an attempt to deal with this problem. The working material is contained in a heated chamber C and some of the vapors allowed to expand into a second chamber D at a slightly higher temperature. From here vapors pass through a multi-channel array E and into the beam chamber. The process of passing through the multi-channel array creates a quasi-collimated beam. The tubes of the collimator array are "bright wall" tubes, that is, any atom or molecule which strikes the wall of the tube must subsequently reevaporate and come back off the wall. Most of the atoms which enter a collimator tube return to the oven, while a smaller number travel the length of the tube and exit as part of the collimated beam. The effect of the "bright walled" tube collimator is to leave the forward directed flux unchanged, but to reduce the total amount of material leaving the oven to: ##EQU2## ps where r is the tube radius and L is its length.
While this device in part solves the excessive emission problem of the oven shown in FIG. 4, it suffers from several problems of its own. The collimation effect for a given aspect ratio (collimator hole area to length) has been reduced from ##EQU3## in the oven of FIG. 4, to ##EQU4## resulting in an increase in the amount of non-useful material injected into the beam area, material which can have long-term detrimental effects. The oven also requires structures to provide anti-spill functions when used in a non-laboratory application, and, with some materials, particularly those of interest to time standards, the small holes of the multichannel array have shown some tendency periodically to plug and unplug, giving rise to a spatially non-uniform and unstable beam.
A third, considerably more complicated, device is disclosed in R. D. Swenumson and U. Even, "Continuous Flow Reflux Oven as the Source of an Effusive Molecular Cs Beam," Rev. Sci. Instrum., 52(4):559-561 (April 1981). This device uses a series of baffles to provide the collimation effect and a steel mesh to provide capillary action to return excess material caught by the baffles to the oven chamber. Its disadvantages include its complexity, and its sensitivity to orientation and acceleration. In addition, its baffles and collimators are susceptible to condensate induced changes in beam shape and even plugging in the absence of gravity.